Writable optical discs may be read or written on by lasers in optical disc drives. Writable optical discs include write-once media, known as WORM (write once, read many), and rewritable media, such as magneto-optical or phase-change media. Such media are usually provided in a cartridge housing (not shown) which protects the media. Alternatively, such discs may be provided without cartridges within a storage system which acts like a jukebox or magazine system by storing and retrieving individual discs within a library of discs.
A typical prior art writable optical disc 10 is shown in FIG. 1. Disc 10 is comprised of a disc-shaped writable medium substrate 12 and a central hub 14. Hub 14, usually comprising a magnetizable metal or metal-filled plastic, is provided at the center of the disc, and defines a central bore 16 through which a drive pin (not shown) of an optical disc drive (not shown) may be inserted. Disc 10 is shown in cross-section in FIG. 2.
As shown in FIG. 2, the outer annular portion of hub 14 is positioned on top of the inner annular portion of substrate 12 so that the combined thickness, t.sub.1, of the hub and the inner annular portion of the substrate is greater than the thickness, t, of the substrate, i.e., t.sub.1 &gt;t.